Broadband access can be implemented by a fiber optical access network, e.g. a B-PON (Broadband Passive Optical Network or a G-PON (Gigabit-capable Passive Optical Network). A passive optical network (PON) does not use any electrically powered components to split the signal, and it comprises an OLT (Optical Line Terminal) located at the service provider's central office and providing an interface for the delivery of the services, e.g. telephony, Ethernet data or video, over the PON. The PON typically comprises one or more ONUs (Optical Network Units) or ONTs (Optical Network Terminations), which are connected to the OLT over an ODN (Optical Distribution Network), the ODN including optical fibers. Each ONT/ONU terminates the PON and converts the optical signals into electrical signals for delivery of the services to the end-user terminals, via a suitable user interface.
A so-called MDU (Multi-Dwelling Unit) is an ONU/ONT for multiple subscribers, e.g. for a number of apartments in an apartment building, and the MDU is provided with a suitable number of ports, such as e.g. 24. On the contrary, a Single Family Unit is an ONT/ONU adapted for a single household.
According to the standards, an ONU (Optical Network Unit) is a generic term for a device that terminates any one of the distributed (leaf) endpoints of an ODN, implements a PON protocol, and adapts PON PDU (Protocol Data Units) to subscriber service interfaces. In some contexts, an ONU implies a multiple-subscriber device, such as the above-mentioned MDU. An ONT (Optical Network Termination) is defined as a single subscriber device that terminates any one of the distributed (leaf) endpoints of an ODN, implements a PON protocol, and adapts PON PDUs to subscriber service interfaces. An ONT may be further provided with an Integrated Residential Gateway.
Thus, an ONT and an MDU are both examples of an ONU. Hereinafter, the generic term ONU denotes an ONU, ONT or an MDU.
The PON management protocol, i.e. the ONT Management and Control Interface (OMCI) protocol, is standardized e.g. according to the OMCI G.984.4-specification, and this management protocol defines different management models. Furthermore, the OMCI-specification contains data models that describe different parts associated with a chassis-based ONU, i.e. a Cardholder, a Circuit Pack and a Port Mapping Package-G.
The Cardholder Managed Entity (ME) defines a cardholder, i.e. a slot, in a chassis where a circuit pack, i.e. a circuit board/card provided with (user) ports, can be inserted, and the Circuit pack Managed Entity defines a circuit pack for insertion into the cardholder. The Port Mapping Package-G defines the ports/user network interfaces (UNI) that exist on a Circuit Pack inserted into a cardholder, but the Port Mapping Package-G is optional, since the Circuit pack ME may also define the ports on a circuit pack. However, in scenarios where a circuit pack contains different ports (UNIs), the Port Mapping Package-G ME is convenient, since it can define multiple port types for a single circuit pack.
Thus, as described above, the Cardholder Managed Entity is defined in the OMCI-specification, and it represents the slot in which a circuit pack can be inserted. This managed entity is created by a chassis-based ONU at startup, based on the pre-defined number of physical slots in the ONU chassis. The Cardholder ME has a number of configuration parameters, such as e.g. expected equipment ID and actual equipment ID, to enable both a configuration of an expected circuit pack in a slot, as well as a notification of an actually inserted circuit pack.
The Circuit Pack ME defines properties for a circuit pack, and it comprises a number of configuration parameters, such as e.g. the actual port numbers, type of ports, serial number, and vendor. This circuit pack managed entity could be created at different instances and circumstances, for example when the OLT provisions an expected equipment ID, or when the Cardholder ME is configured for plug-and-play. The Circuit Pack ME is provided with correct information when a circuit pack is inserted into a cardholder, i.e. a slot in a chassis of an ONU.
The Port Mapping Package-G Managed Entity enables a circuit pack to represent different type of UNIs (User Network Interfaces) on the same circuit pack, which is an extension to the information found in Circuit Pack.
FIG. 1a illustrates a conventional 24 ports Ethernet and Voice capable, integrated and non-modular MDU (Multi-Dwelling Unit) 10, provided with a single optical uplink interface 13a for connecting an optical fiber. If an operator needs more user interfaces, a possible solution would be to stack another MDU to a first MDU and pull another fiber, which is costly. Another solution would be to build a larger MDU, having more end-user ports. However, an operator may prefer to not invest in a larger unit until additional subscribers exist. Instead, several operators are asking for a modular MDU that can be expanded as the number of subscribers increases.
However, a modular MDU/ONU is normally more expensive, since it requires more flexibility, a backplane, a controller card, and removable circuit cards. A non-modular ONU typically has a single optical interface serving all the user interfaces, but cannot be expanded to support an increasing number of end-users.
Since a chassis-based modular ONU comprising removable circuit cards is not always feasible for cost reason, it would be advantageous to implement a modular ONU by stacking additional ONUs to a main ONU without any chassis, enabling an increase of the number of user ports, as the number of subscribers increases. Thereby, a modular ONU can be achieved without requiring any additional optical fibers, or any chassis-based ONU with backplane. The stacked units may be interconnected e.g. using the Ethernet, keeping the same hardware and single software.
Thus, a modular ONU may be implemented by stacking a second MDU 12a to a main MDU 10, with a suitable interconnection 14a between the units, the second unit sharing the PON uplink interface 13a with the main unit, which is illustrated in FIG. 1b. 
FIG. 1c illustrates the stacking of two MDUs 12a, 12b to the main MDU 10, with a first interconnection 14a between the main MDU 10 and the second MDU 12a, a second interconnection 14b between the second MDU 12a and the third MDU 12b, and a third interconnection 14c between the main MDU 10 and the third MDU 12b. The main unit and the third MDU are both provided with PON uplink interfaces 13a, 13b, wherein the resulting stacked MDU may have two PON uplinks for redundancy and/or aggregation. By having two MDUs with a PON uplink interface in the same stacked cluster, the traffic load from the user interfaces could be shared between the two available PON uplinks. If one of the uplinks fails, all the traffic could be re-directed through the working uplink. The available bandwidth would be less then when both units are operating, but it would provide a certain protection when the service level degrades.
However, as described above, the Managed Entities of the OMCI model are defined for a chassis-based ONU that has a pre-defined set of cardholders/slots, with removable circuit packs for insertion into the cardholders/slots. The OMCI-model, as defined in the OMCI specification, does not cover the scenario of simply stacking additional ONUs to a main ONU, without any chassis.